The present invention relates to a method of forming an eccentrically expanded pipe and an eccentrically pipe-expanding device suitable for use in the method.
As shown in FIG. 6, a fuel inlet pipe FP comprises a large-diameter portion FPa, a gradually changing portion FPb, and a small-diameter portion FPc, in which a central axis of the large-diameter portion FPa and that of the small-diameter portion FPc are in eccentric relation to each other. In the fuel inlet pipe FP, the diameter of the large-diameter portion FPa is 1.9 or more times greater than that of the small-diameter portion FPc. Moreover, as mentioned above, the central axis of the small-diameter portion FPc is in eccentric relation to that of the large-diameter portion FPa. For these reasons, it has been difficult to produce the fuel inlet pipe FP by a pipe-expanding method, and it has been usual to produce it by welding three parts, i.e., the large-diameter portion FPa, gradually changing portion FPb, and small-diameter portion FPc.
Now, in order to integrally form such a fuel inlet pipe FP by use of the pipe-expanding method, the following procedure is undertaken.
In general, if a pipe is expanded to such an extent that a limit of expandability of its material is exceeded, an expanded portion of the pipe is cracked or a base pipe portion (non-expanded portion) of the pipe is buckled and, therefore, it is impossible to highly expand the pipe in one process. Accordingly, a fuel inlet pipe to be expanded at a high expansion ratio, such as the fuel inlet pipe FP as shown in FIG. 6, is gradually expanded through a plurality of pipe-expanding operations. Also, in order to place the large-diameter portion FPa and the small-diameter portion FPc in eccentric relation, the small-diameter portion FPc is decentered little by little relative to the large-diameter portion FPa, while the pipe is expanded in stages.
However, in cases where a pipe is eccentrically expanded, the pipe is partially expanded in a large degree because of decentering, and an expansion ratio becomes substantially high in such a portion where the pipe is highly expanded. As a result, even if the pipe is eccentrically expanded little by little in stages, there is still a strong possibility that the pipe is cracked in the portion where it is highly expanded.
The present invention was made to solve the aforementioned problem. More specifically, an object of the invention is to provide a method of eccentrically expanding a pipe in which formation of cracks or the like is prevented even if a desired expansion ratio is high, and in which an eccentrically expansion pipe can be integrally formed. Also, another object of the invention is to provide an eccentrically pipe-expanding device which is suitable for use in the method.
In order to attain the aforementioned objects, there is provided a method for forming an eccentrically expanded pipe, the method comprising a coaxially expanding process in which a portion of a base pipe is expanded by use of an expander punch to form a processed pipe having a neck portion, of which the diameter is the same as that of the base pipe, a tapering portion, and an expanded portion, all of these portions being connected in coaxial relation to one another; and an eccentrically expanding process in which a central axis of the neck portion and a central axis of the expanded portion are decentered relative to each other, and the expanded portion of the processed pipe is further expanded by use of an expander punch having a diameter larger than that of the expander punch used in the coaxially expanding process, thereby forming an eccentrically expanded pipe. In this method, the coaxially expanding process is performed one time or a plurality of times, while the eccentrically expanding process is performed only one time.
In the coaxially expanding process according to the invention, the pipe is coaxially expanded by use of the expander punch, and decentering is not performed in this process. It is preferable to expand the portion of the base pipe in a plurality of stages by use of expander punches of various diameters, in other words, to perform this coaxially expanding process a plurality of times, depending on a desired expansion ratio. As an example, the pipe is preferably expanded at a low expansion ratio (for example, 30 to 55%; specifically, 35 to 50%) in a first coaxially expanding process, and then expanded at a high expansion ratio (for example, 65 to 85%; specifically, 70 to 80%) in a second coaxially expanding process. By expanding the pipe in stages in this manner, the pipe can be safely expanded without being cracked even in cases where the desired expansion ratio is high. According to the Japanese Industrial Standards (JIS), the expansion ratio is represented by the following formula; and in the invention, the expansion ratio is calculated using the diameter of the base pipe as xe2x80x9cDxe2x80x9d in the following formula.                               Expansion ratio                =                                            D1              -              D                        D                    xc3x97          100          ⁢          %                                    [                  Formula          ⁢                      xe2x80x83                    ⁢          1                ]            
D: Outside Diameter of Pipe Before Pipe Expansion
D1: Outside Diameter of Pipe After Pipe Expansion
It is preferable that, in the coaxially expanding process, 80% or more of pipe expansion is performed relative to a desired expansion ratio (that is, an expansion ratio of the expanded portion of the eccentrically expanded pipe). For example, if the desired expansion ratio is 90%, an expansion ratio of the expanded portion of the processed pipe is preferably 70% or more after the coaxially expanding process.
Now, in the eccentrically expanding process according to the invention, the central axis of the neck portion and that of the expanded portion are decentered relative to each other, and the expanded portion of the processed pipe after the coaxially expanding process is further expanded, thereby forming the eccentrically expanded pipe. This eccentrically expanding process is performed only one time, without being divided into a plurality of stages. As mentioned above, the coaxial expansion of the pipe can be achieved in one stage or in a plurality of stages with little possibility of formation of cracks or the like. On the other hand, if the eccentric expansion of the pipe is achieved in a plurality of stages, there is a possibility that cracks may be formed on the pipe since an expansion ratio of a portion of the pipe, which is highly expanded at the time of decentering, becomes substantially high. For this reason, the eccentrically expanding process is performed only one time.
As aforementioned, according to the invention, the multistage eccentric expansion of the pipe is never performed, and instead, the coaxial expansion of the pipe is performed in one stage or in a plurality of stages, and subsequently, the eccentric expansion is accomplished in one stage. As a result, the eccentrically expanded pipe can be formed without any cracks or the like formed thereon.
The method of the invention for forming an eccentrically expanded pipe is suitable, particularly, for making a pipe expanded at a high expansion ratio of 90% or more in its expanded portion. In this case, the pipe is expanded by a plurality of coaxially expanding operations such that the expansion ratio of the expanded portion of the processed pipe becomes 70 to 80% after the coaxially expanding process. And then, the pipe is further expanded such that the expansion ratio of the expanded portion of the eccentrically expanded pipe becomes 90% or more after the eccentrically expanding process. This is a preferable procedure to surely prevent the formation of cracks. In this manner, the present invention makes it possible to integrally form an eccentrically expanded pipe, without forming any cracks or the like thereon, even in cases where the expanded portion of the eccentrically expanded pipe is desired to be expanded at a high expansion ratio such as 90% or more.
The method of the invention for forming an eccentrically expanded pipe is suitable, particularly, for making a fuel inlet pipe. In recent years, a type of fuel inlet pipe called one-inch eccentrically expanded fuel inlet has been developed. The one-inch eccentrically expanded fuel inlet has an expansion ratio of over 90% in its large-diameter portion, and moreover, its small-diameter portion and large-diameter portion are in eccentric relation to each other (see FIG. 6). Therefore, it has been considered to be impossible to integrally form this type of fuel inlet pipe so far; however, it was made possible for the first time by use of the forming method of the invention.
In cases where the method of the invention for forming an eccentrically expanded pipe is applied to the making of the fuel inlet pipe, it is preferable, in the eccentrically expanding process, to dispose HAZ portions in the range of 45 to 135 degrees relative to a direction of eccentricity, seen in section of the pipe. A HAZ portion means a weld heat affected zone (referred to as a HAZ in general). More specifically, it is a portion formed on the periphery of weld metal when flux and part of base metal are fused by heat energy such as an arc.
Since the HAZ portions are different from the other portions in expanding properties, it is not preferable to dispose the HAZ portions in an area where the pipe is highly expanded at the time of decentering, that is, in an upper part of the fuel inlet pipe. This is because there is a possibility that the pipe might be cracked if the HAZ portions are disposed in such an area. On the contrary, it is preferable to dispose the HAZ portions in an area where the pipe is not so highly expanded at the time of decentering, that is, in a lower part of the fuel inlet pipe. However, the HAZ portions are easily cracked by a shock, such as an automobile collision, and therefore, if the HAZ portions are disposed in the lower part of the fuel inlet pipe, fuel leaks may be caused in the case of formation of cracks in the HAZ portions. Consequently, it is preferable to dispose the HAZ portions in the range of 45 to 136 degrees relative to the direction of eccentricity, seen in section of the pipe, such that the HAZ portions can be prevented from being cracked at the time of decentering, and such that even in the event that the HAZ portions are cracked by the automobile collision or the like, fuel leakage from the cracked portions can be prevented.
When the eccentrically expanding process according to the invention is implemented, the following device is preferably used. That is, an eccentrically pipe-expanding device comprising:
neck-portion holding means moveable in a predetermined radial direction while holding a periphery of the neck portion of the processed pipe;
neck-portion moving means for moving the neck-portion holding means in the predetermined radial direction, thereby placing the neck portion and the expanded portion in eccentric relation to each other; and
an expander punch for being pressed, from the expanded portion of the processed pipe held by the neck-portion holding means, into the expanded portion in an axial direction thereof. Use of the eccentrically pipe-expanding device facilitates the eccentrically expanding process of the invention.